Technical Field
Embodiments of the invention relate to pulverized coal boilers and, more particularly, to a system, method and apparatus for controlling the flow distribution of coal between outlet pipes of a pulverizer.
Discussion of Art
Coal fired boilers utilize pulverizers to grind coal to a desired fineness so that it may be used as fuel for burners. In a typical pulverized coal boiler, coal particulate and primary air flow from the pulverizers to the burners through an array of coal pipes leading from the pulverizers to the burners. Typically, raw coal is fed through a central coal inlet at the top of the pulverizer and falls by gravity to the grinding area at the base of the mill. Once ground using one or more of a variety of known methods, the pulverized coal is transported upwards using air as the transport medium. The pulverized coal passes through classifier vanes within the pulverizer. These classifier vanes may vary in structure, but are intended to establish a swirling flow within the classifier and rejects cone to prevent coarse coal particles from flowing into the discharge turret of the pulverizer. The centrifugal force field set up in the rejects cone forces the coarse coal particles to drop back down onto the grinding surface to be reground until the desired fineness is met. Once the coal is ground finely enough, it is discharged from the pulverizer and distributed among multiple pulverized coal outlet pipes and into respective fuel conduits where it is carried to the burners.
With reference to FIG. 1, in a conventional coal pulverizer 10, raw coal is fed into a coal inlet pipe and by force of gravity falls through a centrally located coal chute 12 until it reaches a grinding platform 14 where a grinding mechanism 16 grinds the coal into fine pieces. Air flows into an air inlet port 18, feeding primary air into the pulverizer 10. This creates a stream of air that carries the particles of pulverized coal upward from the grinding platform 14 where they enter classifier vanes 20 of a classifier 22 that establish a swirling flow within the classifier and a reject cone 24. The centrifugal force set up in the reject cone 24 prevents coarse pieces of coal from entering the discharge turret 26, as discussed above. The coarse pieces of coal fall by force of gravity back into the grinding platform 14, to be reground by the grinding mechanism 16 until they reach a desired degree of fineness. The pulverized coal that is not too coarse, however, is directed by the swirling flow of air upwards through a deflector ring 28 of the classifier 22, and into the discharge turret 26 located above the deflector ring 28. Once the pulverized coal enters the discharge turret 26 it is distributed between the multiple pulverized coal outlet pipes 30 (FIG. 1 shows seven pulverized coal outlet pipes at the top of the turret 26). The pulverized coal is then carried by connected fuel conduits (not shown) to a boiler where it is burned as fuel.
While the swirling flow of pulverized coal is efficient in preventing coarse coal particles from being carried upward to the coal pipes, such swirling flow has also been known to create an imbalance in coal flow distribution between the coal pipes 30. As illustrated by the particle tracking diagrams of FIGS. 2-4, the swirling flow created in the classifier 22 also extends into the deflector ring 28 and the turret 26, leading to an imbalanced distribution of coal between the various pipes 30. In particular, as shown in FIGS. 2 and 3, the trajectory 32 of coal particles within the deflector ring 38 has a substantially horizontal component, and only a slight vertical component. The same is true for the trajectory 34 of coal particles within the turret 26. This has been shown to lead to a greater distribution of coal into some of the pipes as compared to others (see, e.g., FIG. 3, where the coal pipe at the bottom right receives a lesser flow of coal particles as compared to the others).
This unbalanced distribution of coal among the coal outlet pipes can adversely affect the performance of each burner and the boiler as a whole and can lead to decreased combustion efficiency, increased potential for tube fouling, furnace slagging, and non-uniform heat release within the combustion chamber. In addition, unbalanced distribution of coal can also result in the inability to control individual burner stoichiometry (i.e., the air-to-coal ratio), which can lead to elevated emissions of nitric oxides, carbon monoxide and the like.
In view of the above, there is a need for a system and method for ensuring a more uniform distribution of coal between the various outlet pipes of a pulverizer in order to improve overall system efficiency and performance.